1. Field of the Invention
The present invention relates to an optical sampling waveform measuring apparatus for measuring an optical waveform in an ultra-short period region where a method using a photoelectric conversion element is impossible. The present invention particularly relates to an optical sampling waveform measuring apparatus which can measure an optical waveform in high timing resolution and in a low timing jitter.
2. Description of Related Art
Conventionally, according to Japanese Examined Patent Application, Second Publication No. Hei. 6-63869, an optical sampling waveform measuring apparatus utilizes a Sum-Frequency Generation (hereinafter called SFG) as a second non-linear optical effect or a Difference-Frequency Generation (hereinafter called DFG) and performs a cross-correlation of an optical pulse having an angular frequency of (ω1) to be measured and a sampling optical pulse having an angular frequency of (ω2) of which the pulse width is narrower than a pulse width of an optical pulse having an angular frequency of (ω1) to be measured in a non-linear optical crystal, and extracts a Sum-Frequency light (hereinafter called SF light) having an angular frequency of (ω1+ω2).
Photoelectric conversion is performed on the SF light by a receptor such as a photoelectric conversion element, and a waveform as a sampling result is displayed by processing the signal electrically.
Timing resolution the SF light waveform in such an measuring apparatus is limited by a pulse width of a sampling optical pulse to be measured and a variation of the sampling pulse in a time-axis such as a timing jitter.
Also, a cycle frequency of the sampling optical pulse (hereinafter called cycle frequency) synchronizes to a value such as 1/n (n is an integral number) of the cycle frequency of an optical pulse to be measured. Also, in order to perform a sampling of an optical pulse to be measured, it is necessary to delay the sampling timing slightly in a range of pulse width of a sampling optical pulse and shift the cycle frequency entirely.
In general, various methods for generating an optical pulse to be used as a sampling optical pulse exist. In every method, several ps of optical pulse width can be obtained, and several hundreds of fs to several ps of timing jitter can be obtained.
Therefore, in an optical sampling waveform measuring apparatus, in order to improve timing resolution, it is necessary to perform an optical pulse compression to narrow the optical pulse width and reduce the varying range of timing jitter in a time-axis.
For example, in a gain switching method in which a cycle frequency can be controlled very easily, optical pulse width of an optical pulse which is generated is nearly 20 ps and the timing jitter is nearly 5 ps. These values cannot be used for a sampled optical pulse for measuring a waveform of signal light to be measured having a frequency which is more than 100 Gb/s because measurement accuracy may be worsened.
Therefore, the above-mentioned optical pulse compression and reduction of time jitter are performed; thus, a sampling optical pulse having 1 ps of optical pulse width and 160 fs of timing jitter is obtained (Referenced document: Development of 310 GHz optical sampling system, Authors: KAWAGUCHI, NOGIWA, OTA, ENDO, Document No. B-10-149 from Society Meeting of The Institute of Electronics, Information, and Communication Engineers).
An example of a structure of an optical sampling waveform measuring apparatus is shown in FIG. 5.
An electrical signal generator SG1 generates, for example, a periodic electrical signal, and outputs a signal P1 to be measured having a frequency fsig as a cycle frequency. An electrical signal generator SG2 generates, for example, a periodic electrical signal, and synchronizes to a signal P1 to be measured so as to generate a sampling signal P2 having a frequency of “(fsig/n)−Δf” (n is an integral number) as a cycle frequency.
An amplifier 100 amplifies an input sampling pulse signal P2 and a narrow pulse generator 101 obtains an electrical pulse having narrower pulse width.
A laser oscillator 102 generates an optical pulse having short pulse width by the above-mentioned electrical pulse with a gain switch method. An optical circulator 103 inputs a continuous light which is generated in a laser oscillator 104 (CW light) to a laser oscillator 102 in order to reduce a timing jitter of the sampling optical pulse, and outputs an optical pulse P3 which is generated by a laser oscillator 102.
A DCF (dispersion compensating fiber) 105 performs a linear compression to above-mentioned optical pulse P3. An EDFA (Erbium-doped fiber amplifier) 106 amplifies the optical pulse P3 which is linearly compressed. A DSF (dispersion shift fiber) 107 extends an inputted optical pulse P3 in a rectangular shape.
Next, an optical amplifier 108 amplifies an optical pulse P3 which is transformed in a rectangular shape and performs a pulse compression and controls a polarization direction of an optical pulse P3 by a polarization direction controller 109 and outputs as a sampling optical pulse P4.
Also, an MLFRL (mode-locked fiber ring laser) 110 synchronizes to a frequency of a signal P1 to be measured and generates an optical pulse P6.
An optical intensity modulator 112 modulates an optical pulse P6 by a predetermined pattern (data row made of 0 (zero) and 1 (one)) which a pattern generator 111 synchronizes to a signal P1 to be measured and outputs an optical pulse P7 which is modulated.
An optical amplifier 113 amplifies an optical pulse P7, and a polarization controller 114 controls a polarization direction of an optical pulse P7 which is input, and outputs as an optical pulse P8.
A polarization beam splitter 115 mixes an optical pulse P8 and a sampling optical pulse P4 and outputs a multiplied optical pulse P9.
A non-linear optical crystal element 116 is made from a non-linear optical member. When a phase matching of a sampling optical pulse P4 and an optical pulse P8 is completed in an optical pulse P9, a non-linear optical crystal element 116 emits an SF light as a cross correlation signal of these two optical pulses.
A receptor 118 is a photoelectric conversion element such as an avalanche photodiode, and performs photoelectric conversion of an input SF light and outputs as a detection signal PS.
An A/D converter 119 converts peak voltage of an input detected signal PS to a digital value according to a predetermined timing and outputs.
A computer 120 performs processing of the above-mentioned digital value, generates an eye-pattern, displays such image of the eye-pattern in a display section, and evaluates the property of a signal light waveform (optical pulse P7) to be used for a communication.
However, in an optical sampling waveform measuring apparatus as shown in FIG. 5, when the optical pulse compression method is used, optical property is unstable because the temperature of a narrow pulse generator 101 as a semiconductor element and a laser oscillator 102 varies. Therefore, there is a problem in the stability of the optical property, and although an optical property of a timing jitter has improved to some degree, it is still unsatisfactory for obtaining better time resolution.
Therefore, in order to obtain an optical sampling waveform measuring apparatus having high time resolution and low timing jitter, a sampling optical pulse having a narrow pulse width and a low timing jitter is necessary. However, according to a current method utilizing optical pulse generating method in which a present semiconductor element, it is difficult to obtain a short optical pulse stably.